Nut harvest is over and, barring another dry winter, the fall rains should be starting up in the Western tree nut production regions. Now is the time when nut producers and their pest control advisors should be developing or fine tuning weed management programs for the upcoming growing season.
Over the past decade, many tree nut growers shifted to weed control programs based on postemergence (POST) herbicides such as glyphosate. Although this strategy has been fairly cost effective in the short term, the need for multiple post treatments, shifting weed populations, new herbicide registrations, and the evolution of glyphosate-resistant weeds have increased interest in residual herbicide programs in recent years.
Why Consider A Residual Herbicide Program?
Residual herbicides (also known as preemergence [PRE], or soil-applied herbicides) are usually applied in the orchard before weeds are present; the material persists in the soil at high enough concentrations for some period of time to control weeds that germinate after the treatment. Conversely, herbicides with primarily POST activity control only weeds that are present at the time of application and do not affect subsequent flushes of germinating weeds. A few important orchard herbicides have both PRE and POST activity.
Residual herbicides, typically applied in the late fall or early winter and incorporated into the soil by winter rains, can provide an important start for a season-long orchard weed management program. With appropriate product selection, application rates, and tank-mix partners, residual herbicides often can extend control of winter and summer annual weeds well into the growing season.
There are several benefits to a chemical weed management program that includes both PRE and POST herbicides either in a tank mix or sequential application during the year. Most herbicides with PRE activity are taken up by the roots or shoots of very small weed seedlings shortly after germination, and affected weeds usually fail to emerge or die shortly after emergence. Because any weeds controlled at the seedling stage by the residual herbicide do not have to be controlled later by POST applications, residual herbicides can sometimes reduce the number of POST applications needed. Additionally, the efficacy of later POST applications is often improved following PRE applications because of better spray coverage in less dense weed canopy. This is especially important for POST herbicides that are not translocated (i.e. “contact” herbicides). PRE herbicides applied during the late fall or early winter when orchard access is more certain also can help keep tree rows clear of weeds during periods where timely POST applications are delayed due to weather or soil conditions later in the winter.
Reducing the number of POST applications applied to the tree rows may increase crop safety due to fewer opportunities for non-selective POST herbicides to come in contact with green bark or foliage due to direct application or drift. With the exception of grass-specific herbicides, POST products registered in orchards have little inherent crop safety; instead, safety depends on placement to minimize tree exposure. Because crop safety is dependent on placement, low hanging branches, rootstock suckers, trunks with green or thin bark, and herbicide drift all can lead to tree injury from POST herbicides.
The recent increase in herbicide resistant weeds is one of the most important reasons to consider a residual herbicide program. The primary recommendation for managing herbicide-resistant weeds is to include multiple herbicide modes of action (MOA) in rotation or in sequence. Because many of the currently available residual herbicides have different MOA than commonly used POST products, residual herbicides can help manage existing herbicide-resistant populations and reduce selection pressure for addition resistant species. However, there are no PRE products with the same MOA as glyphosate; thus, including an appropriate residual herbicide can help to reduce the impact of glyphosate-resistant weeds in tree nut orchards. A listing of common orchard herbicides and mode of action information
can be found at http://wric.ucdavis.edu/PDFs/herbicide_registration_on_horticultural_tree_and_vine_%20crops_2012_01.pdf.
Current Residual Herbicide Research
As part of our ongoing research in orchard and vineyard crops, we conduct field trials each year to evaluate residual herbicides alone and in various tank-mix combinations. Several materials have provided good to exceptional weed control for two to six months after treatment, depending on the weed spectrum present. Recent trials have focused on late applications of tank mixes of residual and POST herbicides to control existing weeds while extending residual control further into the season. This work has highlighted the importance of selecting the right tank-mix partners.
While a complete rundown of our research results is not possible here, this information is routinely presented to California nut growers and pest control advisers at University of California (UC) Cooperative Extension meetings and field days across the state, at industry-sponsored events, at the California Weed Science Society annual meeting, and at the annual UC Weed Day. Many of these results are also available online at the UC Weed Science blog (http://ucanr.org/blogs/UCDWeedScience), the UC Weed Research and Information Center (http://wric.ucdavis.edu) or at my UC lab web page (www.plantsciences.ucdavis.edu/plantsciences_faculty/hanson/main/Recent%20Outreach.html).
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